Sintering is a process necessary to the production of ceramic materials, in which microstructural changes occur in a green-formed ceramic body as a result of applied heat. As a result of sintering, densification and coarsening take place such that a reduction of material volume occurs by decreasing material porosity.
The prior art is replete with different mechanisms by which sintering is effected. These include hot pressing (the simultaneous application of pressure and heat, with heat being generated either inductively or resistively), pressureless sintering (mostly done in batch furnacing with radiant heat produced resistively), reaction sintering (where heat is generated by reaction between two constituents such as silicon and carbon), and hot isostatic pressing (known as "HIP"). One disadvantage to conventional techniques involving furnace heating to a temperature typically between 700.degree.-1800.degree. C. is that because the material is heated from the outside, slow heating is required to keep thermal gradients from inducing stresses in the material that can lead to cracking or breakage. Exposure to high temperatures for long periods can also cause an increase in the grain size of the ceramic, which may lead to degradation of structural properties.
Sintering with electromagnetic fields has been conducted with energy at microwave frequencies. Microwave sintering is fundamentally different from conventional sintering because heating occurs internally of the sample, as opposed to applying heat externally as in the case of a furnace. An example of an apparatus and method for sintering large ceramic articles using microwave energy can be found in U.S. Pat. No. 4,963,709 to Kimrey. As described therein, a 28 GHz, 200 kw gyrotron with variable power output is used as the microwave source. The source is connected to an untuned microwave cavity formed of an electrically conductive housing through an overmoded waveguide arrangement. The part to be sintered is placed in the cavity and supported on a removable high temperature table in a central location within the cavity.
Microwave sintering has an advantage over conventional sintering in that it can reduce both the processing temperature and the processing time for some materials. For example, recent work suggests that microwaves at 2.45 GHz and 28 GHz can cause densification to occur at a lower temperature for a given density than sintering with conventional furnaces. For sintering a zirconia-toughened alumina (ZTA) composite at a microwave frequency of 28 GHz, the ceramic reached about 97% of its theoretical density at a temperature of about 1100.degree. C. In contrast, using a conventional furnace achieved this level with a temperature of about 1500.degree. C. This microwave sintering process has been successfully applied to alumina, zirconia, lithium hydride, and a variety of other ceramic systems, including high-temperature superconducting materials. The reduction in processing time has reduced the grain growth of the material and the migration of impurities to grain boundaries, which could improve the mechanical properties and the reliability of the material.
Another advantage of microwave sintering over conventional sintering is related to the volumetric interaction of the electromagnetic fields with the ceramic. For low-loss materials, the microwaves more uniformly heat the ceramic volume, which means that stresses are much lower than those resulting from thermal gradients produced in the sample by conventional furnaces. This volumetric coupling to the ceramic leads to a higher heating efficiency and faster processing times than those achievable in conventional furnaces. However, because of its small penetration depth, microwave sintering is limited to material with relatively small volumes and/or low dielectric loss tangents.